1. Field of the Invention
The present invention relates to data communications equipment. More particularly, the present invention relates to methods for distinguishing different types of signals received by a PCM modem.
2. State of the Art
With the ever-increasing importance of telecommunications for the transfer of data as well as voice, there has been a strong effort to increase data transfer rates over the telephone wires. In 1994, the ITU-T adopted the V.34 Recommendation (International Telecommunication Union, Telecommunication Standardization Sector Recommendation V.34, Geneva, Switzerland 1994). The V.34 standard and subsequent amendments define modem operating speeds of 28.8 kbps up to 33.6 kbps, and the vast majority of modems being sold today adhere to the V.34 Recommendation. However, with the explosion in the use of the Internet, even at the V.34 transfer rates, downloading of large files available on the Internet can take long periods of time. Thus, even as the V.34 standard was being adopted, there was a thrust to provide additional standards recommendations which would increase data transfer rates even further.
Recognizing that further increases in data rates is theoretically limited where the telecommunication network is an analog system (see C. E. Shannon, xe2x80x9cA Mathematical Theory of Communication,xe2x80x9d Bell System Technical Journal, 27:379-423, 623-656 (1948)), there have been various proposals to take advantage of the fact that much of the telecommunication network is now digital. For example, U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (all assigned to ATandT/Lucent and all of which are hereby incorporated by reference herein in their entireties) all discuss techniques which utilize the recognition that the network is mostly digital in order to increase data transmission rates to 56 kbps and higher. Similarly, Kalet et al., xe2x80x9cThe Capacity of PAM Voiceband Channels,xe2x80x9d IEEE International Conference on Communications ""93, pages 507-511 Geneva, Switzerland (1993) discusses such a system where the transmitting end selects precise analog levels and timing such that the analog to digital conversion which occurs in the central office may be achieved with no quantization error. PCT application number PCT/US95/15924 (Publication WO 96/18261) to Townshend which is hereby incorporated by reference herein in its entirety) discusses similar techniques. All of the disclosures assume the use of PAM (pulse amplitude modulation) digital encoding technology rather than the QAM (quadrature amplitude modulation) currently used in the V.34 Recommendation. The primary difference between the ATandT technology and the Townshend reference is that the ATandT technology suggests exploiting the digital aspect of the telephone network in both xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d directions, while Townshend appears to be concerned with the downstream direction only.
Recently, a new Recommendation for standard was adopted by the ITU-T for the purposes of standardizing a PCM-type modem. The new standard, known as xe2x80x9cV.90xe2x80x9d, which is hereby incorporated by reference herein in its entirety, relates primarily to the transmitter of a PCM-type modem, and relates to a modem which exploits the digital aspect of the telephone network in the downstream direction only. The ITU-T has also recently approved an additional standard known as xe2x80x9cV.92xe2x80x9d which relates to a modem which exploits the digital aspect of the telephone network in both the upstream and downstream directions.
In Section 8.4.1, the V.90 Standard requires the provision of a probing signal; also known in the art as digital impairment learning or xe2x80x9cDILxe2x80x9d. The purpose of the DIL is to give the receiver of the receiving (analog) modem the opportunity to measure network impairments. The measurements and determinations made by the receiving modem are used by the receiving modem in formulating an appropriate constellation for the transfer of data. The constellation formulated by the receiving modem is transmitted back to the transmitting modem according to the format set forth in Section 8.5.2 of the V.90 standard.
While much attention has been paid in the prior art to the transmitters in the V.90 and V.92 modems, it will be appreciated that ability to design an appropriate transmission constellation plays a critical role in producing a high quality modem. In particular, according to V.90, the transmitter transmits 8-bit binary numbers (octets) which correspond to 128 positive and 128 negative xcexc-law or A-law levels. These octets go through the digital network and are finally transformed into analog levels in a digital-to-analog (D/A) converter in the central office. To maximize data rates in the presence of network impairments, an optimal signal constellation must be utilized. Thus, it is necessary to relate (correspond) the transmitted octets to the levels received at the D/A output. This relation or correspondence is accomplished by reference to a translation table. Determination of the translation table is not a trivial task because the digital channel has uncertain parameters and the PCM signal is subjected to both digital and analog distortions including digital attenuation (PAD), robbed bits, etc. However, preparation of an appropriate translation table is critical to the high-quality functioning of the data communications. In addition, the translation table is necessary for generating an appropriate constellation design.
As set forth in previously incorporated Ser. No. 09/238,319, an important step in generating a translation table and constellation design is a determination as to whether the signal being received is an A-law signal or a xcexc-law signal. Some countries (particularly European) utilize A-law encoding in their phone networks, and others (e.g., Japan and the U.S.) use xcexc-law encoding. A few countries (such as South Korea) implement both A-law and xcexc-law encoding in their networks.
As a rule, phone networks have only either A-law encoding or xcexc-law encoding for most domestic calls. However, it is possible that the network between a client modem and a server modem may link the A-law and xcexc-law networks when some domestic or international calls are placed. As a result, the client modem can receive any of four types of signals: a pure xcexc-law signal, a xcexc-law signal which is the result of an A-xcexc conversion, a pure A-law signal, or an A-law signal which is the result of a xcexc-A conversion. In establishing a channel, it is critical that the receiving modem determine whether the signals it is ultimately receiving are A-law (either pure or the result of a xcexc-A conversion) or xcexc-law (either pure or the result of an A-xcexc conversion).
The technology set forth in previously incorporated Ser. No. 09/238,319 is effective and capable of distinguishing between a pure xcexc-law signal and a pure A-law signal. In particular, in previously incorporated Ser. No. 09/238,319 a separation function was introduced:       F1    ⁡          (              n1        ,        n2            )        =            ∑              i        =        n1                    i        =        n2              ⁢          {                                    L            ⁡                          (              i              )                                -                                    2              y                        *                          [                              L                (                                  i                  -                                      16                    ⁢                    y                                                  ]                            }                                      ,            
where L(i) is the i-th positive received level corresponding to transmitted Ucode=i, and y is a positive integer preferably equal to one. For pure A-law levels without noise (and with respect any PAD attenuation), for any n2 greater than n1xe2x89xa733, F1(n1,n2) will be zero. On the other hand, for pure xcexc-law levels without noise and with 0 dB PAD attenuation, F1(n1,n2)=33 (n2xe2x88x92n1+1). According to the preferred embodiment of Ser. No. 09/238,319, the value for the separation function is calculated for any non-robbed-bit signal within the frame, or for the average of non-robbed-bit signals. Then, the value for the separation function is compared to a threshold (e.g., five hundred). If the value of the separation function exceeds the threshold, the signal is determined to be a xcexc-law signal. Conversely, if the value of the separation function does not exceed the threshold, the signal is determined to be an A-law signal.
While pure A-law and pure xcexc-law signals are distinguished by the separation function of Ser. No. 09/238,319, in some circumstances, the separation function is not as effective in distinguishing the results of an A-xcexc conversion and a xcexc-A conversion. In particular, values, at PAD=0 of the function DL(i)=L(i+16)xe2x88x922L(i) for pure A, pure xcexc, A-xcexc conversion, and xcexc-A conversion are seen in FIG. 1. As will be appreciated, the values for the A-xcexc and xcexc-A conversions will not permit effective use of the recited separation function, as the calculated value for the separation function F1(n1,n2) for the A-xcexc conversion can result in values well below the given threshold and result in an improper determination of A-law encoding, while, conversely, the calculated value for the separation function F1(n1,n2) for the xcexc-A conversion can result in values well above the given threshold and result in an improper determination of xcexc-law encoding. These results are caused by the difference between the conversion tables for the xcexclaw and the A-xcexc-law network as well as by the difference between the conversion tables for the A-law network and the xcexc-A-law network. The xcexc-A and A-xcexc conversions are given in Tables 3 and 4 of the ITU-T G.711 Recommendation. Based on these tables, a table listing Ucodes sent and Ucodes received after xcexc-A and A-xcexc conversions can be generated as seen in FIG. 2. As can be seen in FIG. 2, each of the conversions result in some Ucodes not being received, and other Ucodes being duplicated. Ucodes from 81 to 127 are not listed in the table, as the received Ucodes for both columns are equal to the transmitted Ucode.
While tables such as shown in FIG. 2 might be generated for specific PAD attenuations and usable to find the encoding law by means of comparing the received and scaled DIL with the set of transformation tables, such an approach is time consuming and is not particularly reliable in the presence of certain levels of intermodulation distortion (IMD), noise, and other channel impairments. In addition, such an approach is not advisable when a wide range of possible PAD attenuations are considered.
It is therefore an object of the invention to provide methods for determining whether a signal being received at a modem is an A-law or xcexc-law coded signal.
It is another object of the invention to provide methods for distinguishing among pure xcexc-law signals, xcexc-law signals which are the result of A-xcexc conversion, pure A-law signals, and A-law signals which are the result of a xcexc-A conversion.
It is a further object of the invention to provide methods for determining the encoding law of a received signal based solely on collected DIL probing data received by a PCM type modem.
In accord with the objects of the invention, a method for determining the encoding law of an incoming signal generally includes obtaining DIL probing data, generating an ordered table of levels therefrom, using the ordered table of levels to find two functions, and using the two functions to determine the encoding law of the incoming signal.
According to a preferred embodiment of the invention, a first of the two functions (the xe2x80x9cZxe2x80x9d function) is a maximum from the values DL(i) and Th1 which are less than T(i), where Th1 is a threshold value, T(i) is a linear function of the Ucode=i (T(i)=a+bi), and DL(i)=L(i+16)xe2x88x922L(i) with L(i) being the DIL signal corresponding to the transmitted Ucode=i. Preferably, for the Z function, i is chosen from Ucode=80 to Ucode=100, so that there are twenty-one DL(i) values, and the maximum value of the twenty-one DL(i) values which is less than T(i) for that value is chosen as the Z function value. If all DL(i) values are larger than T(i), then the threshold Th1 is taken as the Z function value.
According to the preferred embodiment of the invention, the second of the two functions (the xe2x80x9cQxe2x80x9d function) is defined by:   Q  =                    1        2            ⁢              (                                            ∑                              i                =                i5                                            i                -                i6                                      ⁢                          xe2x80x83                        ⁢                          DL              ⁡                              (                i                )                                              +                                    ∑                              i                =                i5                                            i                =                i6                                      ⁢                          xe2x80x83                        ⁢                          DLm              ⁡                              (                i                )                                                    )              -                  S        m            ⁢                        Ly          2                2            
where DLm(i) is the median value of a windowed group of values for DL(i) where i ranges from i5 to i6, Ly=i6xe2x88x92i5xe2x88x921, and Sm is a mean of a windowed group of slopes, wherein each slope relates to the difference between certain DLm(i) values. Preferably, for the Q function, i5 is chosen to equal 65 and i6 is chosen to equal 80. The Q function is substantially independent of IMD and represents a summation.
The Q and Z functions are used in the preferred embodiment of the invention to determine the encoding law according to a logical analysis. If Z is greater than a certain threshold Th2, the final encoding is xcexc-law encoding. If Z is less than or equal to Th2, then Q is compared to a third threshold Th3. If Q is greater than or equal to Th3, the final encoding is A-law encoding. However, if Q is less than Th3, Z is then compared to the first threshold Th1. If Z is greater than the first threshold Th1, then the final encoding is xcexc-law encoding. If not, Q is compared to a fourth threshold Th4. If Q is less than a fourth threshold then the final encoding is xcexc-law encoding. Otherwise, Q is compared to a fifth threshold Th5. If Q is greater than a fifth threshold Th5 then the final encoding is xcexc-law encoding. Otherwise, the final encoding is A-law encoding. According to the preferred embodiment, the various thresholds are set at: Th2=xe2x88x92100, Th3=820, Th4=xe2x88x92200, and Th5=250.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.